This invention relates to a filtering device and, more particularly, to a filtering device suitable for use for precision filtering in a state submerged in liquid to be filtered. Liquid to be filtered includes water for general uses, drinking water, river water, pool water and chemicals.
Known in the art of filtering device is one which employs a plurality of filter tubes. In this filtering device, a plurality of filter tubes are provided in parallel in a container. Liquid to be filtered is introduced from an inlet provided in one end portion of the container via a pump and filtered through these filter tubes and filtered liquid, i.e., liquid which has passed through the filter tubes, is delivered out of a filtered liquid outlet provided in the other end portion of the container.
For the filter tubes used in the prior art filtering device, filter elements for precision filtering which consists of a porous material or a filtering cloth having densely formed pores are used. In these filtering cloth having densely formed pores of these filter elements, however, pores of these filter elements are blocked by fine powdery materials suspending in the liquid to be filtered in a relatively short period of time after starting of operation of the filtering device with the result that filtering efficiency is rapidly dropped. For this reason, the filtering operation of the filtering device has to be stopped for performing backwashing of the surface of the filter elements and resumed after removing the materials which have blocked the pores of the filter elements.
Backwashing has conventionally been made by supplying pressurized backwash water from an outlet of filtered liquid of a container into a filter tube to remove solids blocking pores of a filter element which constitutes the filter tube.
In the conventional backwashing method, it frequently occurs that when solids have been first dislodged from a part of pores which have been blocked and the part of pores are thereby opened, the backwash water injects from the part of pores which have been opened and the pressure of the backwash water applied to the other pores which have not been opened yet thereby is reduced with the result that the backwashing operation is finished while these pores remain blocked with solids. Accordingly, even if backwashing is repeated periodically, blocking of the filter element increases gradually until at last it becomes necessary to replace the entire filter element and this significantly reduces the efficiency of the filtering device.
It is, therefore, a primary object of the invention to provide a filtering device having an improved backwashing device capable of effectively preventing blocking of a filter element.
Since a filter element of the conventional filtering device causes blocking in a relatively short period of time, stopping of the filtering operation and performing of backwashing must be made frequently with resulting decrease in the filtering efficiency.
It is, therefore, another object of the invention to provide a filtering device which can significantly reduce frequency of stopping of the filtering operation and performing of backwashing in combination with the above described improved backwashing device.